Laser Writes Microlenses on Polyethylene

Electronics isn't the only technology where smaller is better. The same is true for infrared optical systems, where microlenses and diffraction gratings improve system performance. Traditionally, these microelements have been constructed using expensive materials or equipment. Now researchers at Centro de Investigaciones en Optica have demonstrated a method that uses local heating from a CO2 laser to shape polyethylene plates into microelements, potentially enabling the mass production of low-cost infrared micro-optics.

Researchers have demonstrated that direct writing with a laser on polyethylene can produce micro-optical elements, such as this microlens, imaged with an interference microscope. The scratches on the surface of the 300-µm-diameter lens were made with the tip of a surface analyzer. Courtesy of Sergio Calixto.Sergio Calixto, a researcher at the center, and Manuel Ornelas-Rodriguez developed the technique, which they reported in the June issue of Optical Engineering. Calixto explained that they selected polyethylene because it is inexpensive and displays acceptable optical properties in the mid-infrared, specifically at 10.6 µm. Polyethylene also has low softening and melting points, as well as low thermal conductivity.

The researchers built their optical elements out of 0.4- and 0.8-mm-thick plates of polyethylene. To fabricate the microlenses, they shone a CO2 laser beam onto the plates through a 2-mm circular hole. The power density and exposure time of the beam varied from 0.108 to 0.318 W/mm2 and from 0.6 to 5 s, respectively. The resulting lenses displayed surface deviation near zero at the center and less than l/4 at their periphery.

After some optimization of the process, the experimenters were able to construct lenses that successfully focused IR images.

Arrays and gratings

In a variation of the process, the researchers created microlens arrays by focusing the CO2 beam with a ZnSe lens into a smaller spot and moving the substrate under it with an X-Y-Z positioning stage. They also manufactured diffraction gratings with a spatial frequency of 6.3 lines per millimeter by splitting the beam with a germanium beamsplitter and by recombining the components on the plate.

The researchers hope to improve both the material and the manufacturing process. Calixto said that they would investigate the use of more powerful and pulsed CO2 lasers to reduce exposure times, which should reduce the surface deviation caused by heat. "This could give elements with better optical characteristics," he said.